In general, a combined propulsion engine or more simply a "combined engine" is constituted by a propulsion system associating either simultaneously or successively an air-breathing engine and a rocket engine within the same device. In a combined engine, oxygen from the air is used to burn fuel in an air-breathing jet engine during a portion of a flight, whereas during another portion of the flight a different oxidizer, carried on board the vehicle, is used within the rocket engine. The rocket engine may operate on its own, or else during a portion of the flight it may operate simultaneously with the air-breathing jet engine.
Combined engines have already been made for propelling missiles. In this case, a solid fuel engine is used to launch and accelerate the missile. The engine then changes into a ram jet making use of oxygen in the air for cruising flight.
Proposals have already been made to implement combined engines for supersonic transport aircraft, and for shuttles or other space vehicles for the purpose of reducing the mass of oxidizer carried by the vehicle, but no such project has yet been implemented.
In this application to space-going aircrafts or vehicles, air is generally used in gaseous form. It is compressed in a compressor analogous to that of a turbojet and it feeds a combustion chamber in which fuel is burnt. The hot gases are then ejected via a nozzle.
Beyond a certain speed, and providing the geometry of the air stream is appropriately adapted, the engine operates as a ram jet (its compressor then "freewheeling"). For non-air-breathing operation, the engine includes a combustion chamber of the type found in a rocket engine together with various pumps and other accessories for feeding the chamber both with fuel and with oxidizer. This chamber is situated inside the air stream of the turboramjet and behind its compressor-turbine portion. The nozzle from this combustion chamber makes use of components that it shares with the nozzle of the air-breathing stream.
The drawback of this device lies in the fact that in operation in turbo jet mode or in ram jet mode, the rocket combustion chamber must be closed by means of a streamlined fairing so as to avoid disturbing flow in the air-breathing stream. In addition, if the fairing is ejected for rocket mode operation, then it is not possible to return to turbo jet or ram jet mode without a major maintenance operation requiring a new fairing to be installed. Finally, the requirement to be able to adapt the section of the throat in the air-breathing nozzle means that the nozzle must be capable of considerable displacement relative to the central body which then acts as a section-varying needle or plunger.
Proposals have also been made, in Document JP-A-52-56209 to provide an air-breathing rocket propulsion system using liquid propellants, the system comprising at least one annular rocket chamber with a projecting tip and placed upstream from the combustion stream of a ram jet, i.e. upstream from the throat of the ram jet nozzle, thereby enabling combined operation to take place in which an entrainment effect occurs on the air taken from outside, with this air being mixed with the initially supersonic gases of the rocket nozzle associated with the rocket chamber. The ejection pressure of the rocket nozzle is thus brought close to that of the air in the mixing zone, and the geometry of the rocket nozzle enables the stream of air and the stream of gases ejected by the rocket to travel in parallel while mixing them with a minimum of losses. In this embodiment, the tapering central body of the spike type delimiting the annular rocket chamber is terminated in a zone of subsonic flow upstream from the throat of the ram jet nozzle. Only two modes of operation are possible: rocket mode only, and combined ram jet and rocket mode.
The present invention seeks to remedy the above-mentioned drawbacks and to provide a combined propulsion engine capable of operating in five different operating modes, namely: air-breathing mode, non-air-breathing mode, and combined mode, with air-breathing mode being either turbo jet mode or ram jet mode, the engine being capable of changing from one mode of operation to another in reversible manner without transient modes giving rise to significant excess stresses and without it being necessary to eject any fairing or the like, with each operating mode being adjustable to flight conditions.
The present invention also seeks to provide a combined propulsion engine having satisfactory efficiency in each of its possible operating modes and enabling thrust to be adapted on a continuous basis to mission requirements and to flight conditions regardless of altitude.
The invention also seeks to make it possible to adapt thrust on a continuous basis to requirements, and in particular to make it possible to operate simultaneously in rocket mode and in turbo jet mode in order to obtain maximum thrust, e.g. during take-off or when changing gradually from air-breathing mode to non-air-breathing mode so as to ensure continuous and constant thrust on leaving the dense layers of the atmosphere.
Another object of the invention is to guarantee continuity in the streams of gases in combustion and to guarantee flow mixing with minimum turbulence for the various possible modes of operation so that in spite of its highly adaptable nature the engine retains high efficiency.